IMBL Monochromator

A Dual Laue Crystal Monochromator (DLCM) is the primary monochromating optic used for imaging and computed tomography (CT) beams. It uses asymmetrically cut thin silicon crystals and cover the range 20 to ~120 keV using the [111] reflection (7.11° to 1.1° Bragg angle) (or the [113] reflection for higher energies (120keV).)

The asymmetry angle will be 14.75° for [111] and -14.75° for [113]. (The angle between [111] and [113] is 29.50°. ). The beam deflection will be upwards with a variable offset of 20 to 25mm and the corresponding horizontal separation of the two crystals will vary from approximately 80 to 630mm.

Some rocking-curve data and X-ray images for the doubly-diffracted beam from the DCLM were collected to facilitate comparison with the proposed calculations. The experiments included running the synchrotron at low current so that effects associated with the 1st-crystal “heat bump” could be avoided (increasing the upstream, in-vacuo, filtration will also achieve this but may well enhance any harmonic contamination as well).

Experimental details are as follows:

3.033GeV. Ring currents of nominally 1, 10, 50 & 200mA. SCMPW operating at 3.0T

in-vacuo filtration:

• 0.45mm graphene

• 21.2mm high-density graphite

• 2.83mm Al

DCLM:

middle of tank at 16.15m from source (crystal horizontal positions can be supplied as needed - 1st crystal position varies with X-ray energy). 20mm vertical offset between crystals. Diffracting section/window of Si crystals is 1mm thick (effective thickness as seen by X-ray beam varies with X-ray energy) and 6mm “high” (other dimension in plane of diffraction) - Si base (with notches) is 25mm thick - length of crystals is 80mm (perpendicular to the plane of diffraction and to the plane of the pictures above)

Crystal Bends:

The absolute value of CB is the ratio of the source-to-DCLM distance to the radius of curvature - CB > 0 if the crystal is concave as viewed from the source (< 0 for convex bending and = 0 for no bending). Standard bending parameters (CB values) are -2 and -1.4 for the 1st and 2nd crystals respectively.

X-ray beam exits 1st crystal via large, flat face (opposite to picture in first figure above) and enters 2nd crystal by the same face (as pictured above)

Data collected in Hutch 2B:

Be window at 32.8m from source (includes short He path and Al foil window)

1st and 2nd ion chambers (ICs) located at 33.6 and 34.6m resp.

ex-vacuo Cu filter placed in between the ion chambers as a check on harmonic contamination of the beam

X-ray images collected on upstream faces of both ICs using EBT3 gafchromic film.

1. See rocking-curve data for actual ring-current values.

2. 33.2keV corresponds to the Iodine K-edge.

3. 1^st no. is rocking curve of 1^st crystal; 2^nd no. (when provided) is for rocking curve of 2^nd crystal.

4. Calculated from 1^st & 2^nd IC currents at the peak of the 1^st crystal rocking curve; theoretical values (including allowance for 1m air path between ICs) are 0.462 for 33.2keV/0.1mm Cu, 0.476 for 70.0keV/0.75mm Cu, & 0.488 for 100.0keV/ 1.7mm Cu.

5. Scaled to 200mA ring current.

6. Integrated flux values are calculated from 1^st IC current at the peak of the 1^st crystal rocking curve (values in previous column).  IC electron-loss corrections are included (corrections for recombination and other effects can be neglected here); IC plate length = 5cm; Q = 1.6022´10^-19C; W_air = 33.97eV; ρ_air = 0.001205g/cm³; mass energy-absorption coefficients are taken from the NIST database.

7. Calculated from 5^th and 9^th columns.